Manufacturing processes are generally sensitive to variations in process parameters. This is especially true in the semiconductor manufacturing industry. For example, during the fabrication process for integrated circuits on silicon substrates, gas flow for diffusion, dry etch and implant processes must be maintained within a narrow range by a mass flow controller; the output from a radio frequency generator used to excite gasses during dry etch, chemical vapor deposition (CVD) and sputter processes must be precisely monitored; and the pressure of gasses used for diffusion, dry etch and CVD processes must be kept within optimum parameters if commercially-viable wafers yields are to be obtained.
Unfortunately, myriad factors such as heat, aging and failure of components and power surges intervene to disrupt the stability of physical processes. To prevent a process parameter from deviating from an acceptable range, it is essential that an operator be informed when an unacceptable deviation occurs so that the parameter may be adjusted or the process terminated until the parameter can, once again, be brought within the acceptable range.
Heretofore, monitoring of a process output signals generally required the use of complicated analog to digital circuits which could be interfaced to a control computer. It would be highly desirable to have a relatively simple, easily adjustable circuit for monitoring an output representative of a critical process parameter that would provide a signal whenever the output departed from a present range.